A 28-year-old male farmer from Tamil Nadu presents with a 7-day history of fever, severe headache, and myalgia. On examination, he has a temperature of 39.5°C, a characteristic black eschar on his left thigh, and generalized lymphadenopathy. A rash appeared on day 5 of illness, starting on the trunk and spreading to the extremities. Laboratory findings show thrombocytopenia (platelets 85,000/µL) and elevated transaminases (AST 120 U/L, ALT 95 U/L). What is the most likely diagnosis?

Explanation

## Clinical Diagnosis: Scrub Typhus ### Key Clinical Features **Key Point:** The characteristic black eschar (tsutsugamushi lesion) is pathognomonic for scrub typhus. It appears at the site of mite bite and is present in 50–80% of cases. ### Diagnostic Triad | Feature | Scrub Typhus | RMSF | Murine Typhus | |---------|--------------|------|---------------| | **Eschar** | Present (50–80%) | Absent | Absent | | **Rash onset** | Day 4–6 | Day 2–4 | Day 5–8 | | **Rash distribution** | Trunk → extremities | Wrists/ankles → trunk | Trunk-sparing | | **Lymphadenopathy** | Marked, regional | Absent or mild | Mild | | **Thrombocytopenia** | Common | Common | Mild | | **Vector** | Trombiculid mite | Tick | Flea | | **Geography** | SE Asia, India | USA, Americas | Worldwide | ### Clinical Pearl **Clinical Pearl:** Scrub typhus is endemic in the "tsutsugamushi triangle" (Japan to Pakistan to Australia), with India being a major endemic zone. The eschar is the single most specific finding. ### High-Yield Features in This Case **High-Yield:** - Black eschar on thigh (mite bite site) - Regional lymphadenopathy (draining lymph nodes) - Rash appearing on day 5 with trunk-to-extremity spread - Thrombocytopenia and transaminitis (hepatitis common) - Farmer with exposure to mite-infested vegetation ### Pathophysiology Scrub typhus is caused by *Orientia tsutsugamushi* (formerly *Rickettsia tsutsugamushi*), transmitted by trombiculid mite larvae. The eschar forms at the inoculation site due to local vasculitis and necrosis. ### Treatment Doxycycline 100 mg BD for 7 days (or azithromycin in pregnancy/children) — response within 48 hours is expected. [cite:Park 26e Ch 3]